This invention relates to a method of bleaching cellulose pulp, and more particularly, to a method of bleaching an alkaline aqueous slurry of cellulose pulp with oxygen, or peroxide.
Cellulose pulp, which is used for many purposes, usually requires bleaching for the removal of any colored substance therefrom, except when used for purposes not calling for its whiteness, as when it is used for a packing material, such as the core of corrugated cardboard.
It has hitherto been usual to use chlorine-containing bleaching agents, such as chlorine, sodium hypochlorite, sodium chlorite, and a bleaching powder, for bleaching such pulp, simply because they are inexpensive and high in reactivity.
These chlorine-containing bleaching agents exhibit an excellent bleaching action, but their reaction with lignin produces harmful substances including organic chlorine compounds, such as dioxin, as by-products, and thereby presents the problem of environmental pollution. Under these circumstances, it has been a matter of great importance for mankind to have the pulp manufacturing industry decrease, or eventually end the use of chlorine.
It has been becoming common practice on a worldwide basis to employ oxygen gas, or hydrogen peroxide as a bleaching agent in a part of a process for bleaching pulp in order to decrease the use of chlorine. It has, however, been impossible to realize a satisfactory reduction in the use of chlorine, since it has generally been impossible to bleach pulp satisfactorily with oxygen, etc. Attempts have been made to employ high-pressure oxygen gas at a high temperature to obtain an improved bleach of pulp by bleaching with oxygen, while decreasing the use of chlorine even by a small amount, but have presented a new problem, since the depolymerization of cellulose is promoted, and results in pulp of lower strength.
In order to solve the above problem, the applicant of this application has previously proposed a method of realizing an improved bleach of pulp by adding a bleaching assistant during its bleaching with oxygen, hydrogen peroxide, etc. (Japanese Patent Application Laid-Open No. Hei 5-186987).
The above method has been able to realize an improved bleach of pulp as compared with any prior method of bleaching with oxygen, etc., but has still been unsatisfactory, and unable to realize a satisfactory reduction in the use of chlorine. Moreover, the heavy foaming of a bleached pulp slurry has presented a problem of operation during the step of dewatering, etc.
We, the inventors of this invention, have made a serious study of a problem as to how it will be possible to achieve an improved bleach of cellulose pulp and thereby a greater reduction in the use of any chlorine-containing bleaching agent by employing oxygen or peroxide, while having cellulose retain its strength, and without having any problem of operation caused by the foaming of a bleached pulp slurry, and as a result, we have made this invention by finding that the use of a specific compound as a bleaching assistant provides a very effective and economical solution to the above problems.
This invention relates to a method of bleaching cellulose pulp, and employs one or more compounds selected from a compound (U) represented by general formula (1), a compound (V) represented by general formula (2) and a compound (W) represented by general formula (3) as a bleaching assistant when bleaching an alkaline aqueous slurry of cellulose pulp (pulp slurry) with oxygen or peroxide:
R1xe2x80x94Oxe2x80x94[(C2H4O)m/(AO)n]xe2x80x94Hxe2x80x83xe2x80x83(1)
R2xe2x80x94Oxe2x80x94[(C2H4O)m/(AO)P]xe2x80x94(AO)qxe2x80x94Hxe2x80x83xe2x80x83(2)
(R3)txe2x80x94Xxe2x80x94[(C2H4O)m/(AO)p]xe2x80x94(AO)rxe2x80x94Hxe2x80x83xe2x80x83(3)
where R1 stands for a branched alkyl group having 6 to 12 carbon atoms, m stands for an average added mol number of 4 to 15, A stands for a propylene, butylene or phenylethylene group, n stands for an average added mol number of 0 to 4, the addition shown in [ ] is in a random or block form, R2 stands for a linear or branched alkyl group having 6 to 12 carbon atoms, p stands for an average added mol number of 0 to 3.9, q stands for an average added mol number of 0.1 to 4, R3 stands for a hydrogen atom, a methyl or ethyl group, or a branched alkyl group having 3 to 12 carbon atoms, t stands for an integer of 1 to 5, X stands for a residual phenol group, and r stands for an average added mol number of from 0, exclusive, to 4, inclusive.
It is possible that R1 in general formula (1), R2 in general formula (2) and R3 in general formula (3) may be a branched alkyl group represented by general formula (4): 
where R4 stands for a methyl, ethyl, propyl or butyl group, R5 stands for a linear or branched alkyl group having 1 to 9 carbon atoms, and R6 stands for an alkyl group having 1 or 2 carbon atoms.
The compounds (U) to (W) preferably have a cloud point of 40xc2x0 C. to 110xc2x0 C. in a 2% aqueous solution.
Bleaching is usually performed with a chlorine-containing bleaching agent before bleaching with oxygen or peroxide, or thereafter. Bleaching with a chlorine-containing bleaching agent is preferably performed after bleaching with oxygen or peroxide.
The use of the bleaching assistant according to this invention makes it possible to realize an improved bleach of pulp and thereby a reduction in the use of a chlorine-containing bleaching agent, while having cellulose retain its strength. The use of the compound (V) as the bleaching assistant is particularly effective for restraining the foaming of a bleached pulp slurry and eliminating any problem of operation caused by its foaming.
[Compound (U)]
The compound (U) according to this invention is obtained by adding ethylene or propylene oxide, etc. to branched alcohol, and represented by general formula (1):
R1xe2x80x94Oxe2x80x94[(C2H4O)m/(AO)n]xe2x80x94Hxe2x80x83xe2x80x83(1)
where R1 stands for a branched alkyl group having 6 to 12 carbon atoms, A stands for a propylene, butylene or phenylethylene group, m stands for an average added mol number of 4 to 15, n stands for an average added mol number of 0 to 4, and the addition shown in [ ] is in a random or block form.
If the branched alkyl group is R1 has less than 6, or more than 1 carbon atoms, no satisfactorily improved bleach of pulp can be obtained. The branched alkyl group R1 is preferably represented by general formula (4): 
where R4 stands for a methyl, ethyl, propyl or butyl group, R5 stands for a linear or branched alkyl group having 1 to 9 carbon atoms, and R6 stands for an alkyl group having 1 or 2 carbon atoms. 
Specific examples of the branched alkyl group R1 represented by general formula (4) are isohexyl, isoheptyl, isooctyl, 2-ethylhexyl, an isobutylene dimer, isononyl, a propylene trimer, isodecyl, isoundecyl, isododecyl, a propylene tetramer, and a branched alkyl group synthesized by the oxo process. A 2-ethylhexyl or isodecyl group is, among others, preferred.
Referring to (C2H4O)m in general formula (1), C2H4O stands for ethylene oxide, and if its average added mol number m is less than 4, or more than 15, no satisfactorily improved bleach of pulp can be obtained. xe2x80x9cC2H4Oxe2x80x9d is sometimes expressed as xe2x80x9cCH2CH2Oxe2x80x9d.
Referring to (AO)n in general formula (1), AO stands for one or more of propylene, butylene and styrene oxides, n stands for the total of the average added mol numbers of those oxides, and if its average added mol number n is larger than 4, no satisfactorily improved bleach of pulp can be obtained.
Referring to (C2H4O)m and (AO)n in general formula (1), there is no rule concerning the order in which C2H4O and AO are added. The compound (U), however, excludes any compound having propylene, butylene or styrene oxide added in a block form to the end.
Specific examples of the compound (U) include the following:
Example 1: Isodecyl-Oxe2x80x94(EO)10xe2x80x94H
Example 2: 2-Ethylhexyl-Oxe2x80x94(EO)12xe2x80x94H
Example 3: 2,4-Dimethylheptyl-Oxe2x80x94[(EO)9//(PO)2]xe2x80x94H
Example 4: Isononyl-Oxe2x80x94[(EO)10//(PO)1//(BO)0.5]xe2x80x94H
Example 5: Isodecyl-Oxe2x80x94(EO)4xe2x80x94(BO)1xe2x80x94(SO)1xe2x80x94(EO)5xe2x80x94H
Example 6: 2-Ethylhexyl-Oxe2x80x94(PO)1xe2x80x94(EO)8xe2x80x94H
In the above formulas (and also in the formulas which will appear below), EO stands for ethylene oxide, PO stands for propylene oxide, BO stands for butylene oxide, SO stands for styrene oxide, the number written at the right of each stands for the average added mol number, // stands for addition in a random form, and xe2x80x94 stands for addition in a block form.
A 2% aqueous solution of any compound (U) preferably has a cloud point of 40-110xc2x0 C., and more preferably, 60-100xc2x0 C.
[Compound (V)]
The compound (V) according to this invention is a modified form of the compound (U), and more particularly, it is obtained by modifying the group selected as R1 in the compound (U), and adding propylene oxide, etc. in a block form to the end, and is represented by general formula (2):
R2xe2x80x94Oxe2x80x94[(C2H4O)m/(AO)p]xe2x80x94(AO)qxe2x80x94Hxe2x80x83xe2x80x83(2)
where R2 stands for a linear or branched alkyl group having 6 to 12 carbon atoms, A stands for a propylene, butylene or phenylethylene group, m stands for an average added mol number of 4 to 15, p stands for an average added mol number of 0 to 3.9, q stands for an average added mol number of 0.1 to 4, and the addition shown in [ ] is in a random or block form.
If the alkyl group R2 has less than 6, or more than 12 carbon atoms, no satisfactorily improved bleach of pulp can be obtained. Examples of the linear alkyl group R2 are n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl and n-dodecyl. Examples of the branched alkyl group R2 are equal to the examples listed before as the branched alkyl group R1 in general formula (1).
(C2H4O)m in general formula (2) stands for the same as it does in general formula (1), and its average added mol number m has its range limited for the same reason.
Referring to (C2H4O)m and (AO)p in general formula (2) there is no rule concerning the order in which C2H4O and AO are added.
Referring to (AO)p and (AO)q in general formula (2), AO stands for one or more of propylene, butylene and styrene oxides, as it does in general formula (1). Thus, its average added mol number p (and also q) means the total of the average added mol numbers of the oxides.
If (AO)q in general formula (2) has an average added mol number q which is smaller than 0.1, the compound is less effective in restraining the bubbling of a bleached pulp slurry than when it has an average added mol number q of 0.1 to 4.
The sum of the average added mol numbers p and q [or the total of the average added mol numbers of propylene, butylene or styrene oxide in general formula (2)] is from 0.1 to 4, and if the sum is larger than 4, no satisfactorily improved bleach of pulp can be obtained.
Specific examples of the compound (V) include the following:
Example 1: Isooctyl-Oxe2x80x94(EO)8xe2x80x94(PO)0.5xe2x80x94H
Example 2: Tetrapropylene-Oxe2x80x94(EO)11xe2x80x94(SO)1.5xe2x80x94H
Example 3: n-octyl-Oxe2x80x94(EO10xe2x80x94(PO)1xe2x80x94H
Example 4: Isooctyl-Oxe2x80x94(EO)12xe2x80x94(PO)0.5xe2x80x94(BO)1.5xe2x80x94H
Example 5: Isononyl-Oxe2x80x94[(EO)5//(PO)0.3//(SO)1]xe2x80x94(PO)0.2xe2x80x94(BO)0.2xe2x80x94H
Example 6: 2,4-Dimethylheptyl-Oxe2x80x94[(EO)6//(BO)0.5]xe2x80x94(PO)1xe2x80x94H
Example 7: 2-Ethylhexyl-Oxe2x80x94(EO)4xe2x80x94(PO)11xe2x80x94(EO)5.5xe2x80x94(BO)0.5xe2x80x94H
Example 8: Isodecyl-Oxe2x80x94(BO)1xe2x80x94(EO)12xe2x80x94(BO)1xe2x80x94H
Example 9: n-decyl-Oxe2x80x94(PO)0.5xe2x80x94(EO)10xe2x80x94(PO)0.5xe2x80x94(SO)0.5xe2x80x94H
A 2% aqueous solution of compound (V) preferably has a cloud point of 40-110xc2x0 C., and more preferably, 60-100xc2x0 C., as is the case with compound (U).
[Compound (W)]
The compound (W) according to this invention is a modified form of compound (V), and more particularly, it is obtained by modifying the group selected as R2 in compound (V), and substituting a residual phenol group for its xe2x80x94Oxe2x80x94 component, and is represented by general formula (3):
(R3)txe2x80x94Xxe2x80x94[(C2H4O)m/(AO)p]xe2x80x94(AO)rxe2x80x94Hxe2x80x83xe2x80x83(3)
where R3 stands for a hydrogen atom, a methyl or ethyl group, or a branched alkyl group having 3 to 12 carbon atoms, t stands for an integer of 1 to 5, m stands for an average added mol number of 4 to 15, A stands for a propylene, butylene or phenylethylene group, p stands for an average added mol number of 0 to 3.9, r stands for an average added mol number of from 0, exclusive, to 4, inclusive, and the addition shown in [ ] is in a random or block form.
If R3 in general formula (3) is a branched alkyl group having less than 3, or more than 12 carbon atoms, no satisfactorily improved bleach of pulp can be obtained. Examples of the branched alkyl group may be equal to those listed before as the branched alkyl group R1 in general formula (1).
(C2H4O)m in general formula (3) stands for the same as it does in general formula (1), and its average added mol number m has its range limited for the same reason.
Referring to (C2H4O). and (AO)p in general formula (3), there is no rule concerning the order in which C2H4O and AO are added.
Referring to (AO)p and (AO)r in general formula (3) AO stands for one or more of propylene, butylene and styrene oxides, as it does in general formula (1). Thus, its average added mol number p (and also r) means the total of the average added mol numbers of the oxides.
Referring to (AO)r in general formula (3), its average added mol number r is preferably from 0.1 to 4.
The sum of the average added mol numbers p and r [or the total of the average added mol numbers of propylene, butylene or styrene oxide in general formula (3)] is larger than 0, but not larger than 4, and if the sum is larger than 4, no satisfactorily improved bleach of pulp can be obtained.
[Bleaching method]
The pulp to which the bleaching method of this invention is applicable is, for example, chemical pulp [such as CGP (chemiground pulp), SCP (semichemical pulp), SP (sulfite pulp), KP (kraft pulp) or AP (alkali pulp)], mechanical pulp [such as GP (ground pulp), RMP (refiner mechanical pulp) or TMP (thermomechanical pulp)], or pulp of used paper, and may be wood pulp [such as pulp of wood of a needle-leaf or broadleaf tree, or of a mixture of wood of needle-leaf and broadleaf trees], or non-wood [herbal] pulp. The pulp may be of the unbleached, or somewhat bleached type.
Sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, ammonia, a kraft-process white liquor, an oxidized white liquor, a green liquor, an oxidized green liquor, sodium tetraborate, sodium metaborate, etc. can be used as an alkaline medium for preparing an alkaline aqueous slurry of pulp (pulp slurry) for the purpose of this invention, though sodium hydroxide is usually used. The amount of the alkaline medium to be added based on the oven-dry weight of pulp is preferably from 0.5 to 3% by weight in terms of sodium oxide (Na2O). The pulp content (by oven-dry weight, also in the following description) of the pulp slurry (or suspension) may be either low (below 10% by weight), or medium (10 to 20% by weight), or high (above 20% by weight).
When oxygen is used for the bleaching method of this invention, either oxygen gas, or oxygen-enriched air may be used, but the use of oxygen gas is preferred in view of the volume of a reaction vessel, its reaction efficiency, etc. The oxygen may be of any pressure from high (10 kg/cm2G or above) to low (below 10 kg/cm2G), but is preferably of a relatively low pressure (from 1 to below 10 kg/cm2G) in order to prevent the promoted depolymerization of cellulose from causing a lowering of pulp strength.
Hydrogen peroxide, peroxy acid [e.g. peracetic acid, percarbonic acid, perboric acid (perborate)], chlorine dioxide, ozone, etc. can be used as the peroxide for the bleaching method of this invention.
The compounds (U) to (W) used as the bleaching assistant according to this invention are usually added in the step of bleaching pulp, but are sometimes added during the preceding step of digesting wood chip, or thereafter. One of the compounds (U) to (W), or an adequate mixture of two or all of them is added. The amount of the compounds (U) to (W) to be added depends on the whiteness required of pulp as bleached, etc., but is usually from 0.1 to 2% by weight based on the oven-dry weight of pulp in its slurry. If it is smaller than 0.1% by weight, no satisfactorily improved bleach of pulp can be obtained, but if it exceeds 2% by weight, no further improvement of bleach can be obtained because of an equilibrium. The use of the compounds (U) to (W) is economical, since they are generally recycled with washings after the step of bleaching pulp, and it is only a very small amount that has to be supplied to maintain the proportion as stated above.
The bleaching assistant according to this invention does not preclude the use of e.g. an alkaline earth metal salt such as a magnesium compound, a chelating, or other agent for preventing cellulose from having a lower degree of polymerization, or a stabilizer for peroxide bleaching, such as a condensed phosphate, or silicate.
A reactor of any type, such as vertical or horizontal, can be used as a bleaching apparatus for carrying out the bleaching methods of this invention. The bleaching assistant according to this invention is usually added through the bottom of an upflow type bleaching column, and though it may be added either before pulp is mixed with oxygen gas, or peroxide, or simultaneously therewith, or thereafter, it is preferable from the standpoint of stirring efficiency to add it simultaneously, or before pulp is mixed with oxygen, or the like. A temperature of from room temperature to 200xc2x0 C. and a time of from 30 seconds to 20 hours are usually employed for the bleaching treatment.
The following are the conditions which are usually employed for (1) oxygen bleaching with oxygen gas, (2) ozone bleaching, and (3) hydrogen peroxide bleaching, when an alkaline aqueous slurry of pulp has a medium pulp content (10 to 20% by weight):
(1) Oxygen Bleaching:
Pulp content: 10-14% by weight, oxygen concentration: 10-30 kg/ton of pulp by oven-dry weight, amount of alkaline medium added: 20-30 kg/ton of pulp by oven-dry weight, amount of magnesium sulfate added: 0-2 kg/ton of pulp by oven-dry weight, bleaching temperature: 90-120xc2x0 C., time: 50-60 min., column top pressure: 3.5-5 kg/cm2G;
(2) Ozone Bleaching:
Pulp content: 10-15% by weight, ozone concentration (based on oven-dry weight of pulp): 0.4-2.0% by weight, bleaching temperature: 20-70xc2x0 C., time: 5-30 min.;
(3) Hydrogen Peroxide Bleaching:
Pulp content: 10-20% by weight, hydrogen peroxide concentration (based on oven-dry weight of pulp): 0.2-2% by weight, bleaching temperature: 60-80xc2x0 C., time: 120-240 min.;
Bleaching with a common chlorine-containing bleaching agent (e.g. chlorine gas, or hypochlorous acid) is usually performed before or after the step of oxygen gas, or peroxide bleaching as described above. Moreover, enzymatic bleaching may sometimes be combined with it.